1. Field of the Invention
This invention relates to phase detection circuits, and more particularly to a phase detection circuit functioning as a time base corrector for correcting the phase shift caused by fluctuation in the time base of analog video signals.
2. Description of the Related Art
Video signal processing devices include video cassette recorders (VCR) using cassettes as recording media and laser disc devices using optical discs as recording media. While video information recorded on the recording medium is being reproduced, variations in the relative speed between the magnetic tape and the head in the VTR may occur. Likewise, in the laser disc device, variations in the relative speed between the pits recorded on the optical disc and the optical pickup may occur.
In order to precisely reproduce the recorded video signals, the video signal processing device generally includes a time base corrector. The time base corrector is divided into two types according to its correction process. One type of time base corrector performs the process of detecting the frequency and phase of a sync. signal included in the reproduced signal and mechanically controlling the rotation speed of the head, tape speed, or the rotation speed of the optical disc based on the detected frequency and phase. The other type of time base corrector effects the process of detecting the frequency and phase of a sync. signal included in the reproduced signal and electrically controlling the delay time of the reproduced signal itself.
The latter type time base corrector includes a triangular wave generation circuit for receiving a reference clock signal having a frequency of 3.58 MHz and generating a triangular wave signal for each period of the clock signal, and a phase comparator circuit for comparing the reference clock signal with a burst signal included in the input video signal to be phase-corrected. A sample-hold circuit samples the triangular wave signal and hold the signal level thereof in response to a sampling pulse which is generated from the phase comparator circuit according to the result of comparison. That is, the sample-hold circuit supplies an error voltage corresponding to the phase difference between the burst signal and the reference signal to a phase shifter circuit. Then, the time base variation of the input video signal can be corrected by the phase shifter circuit.
The conventional time base corrector uses a triangular wave having a frequency of 3.58 MHz. However, it is technically difficult to generate a triangular wave having such a high frequency at high precision, and therefore is becomes difficult to derive a precise error voltage. Further, the conventional time base corrector is sensitive to variations in the power source voltage and temperature variation, and the operation thereof is unstable.